The present invention relates to the field of imaging samples with radiation in the infra-red (IR) and Terahertz frequency range and specifically using radiation in the higher Gigahertz (GHz) and the Terahertz (THz) frequency ranges. In this field, all such radiation is colloquially referred to as THz radiation, particularly that in the range from 25 GHz to 100 THz, more particularly that in the range of 50 GHz to 84 THz, especially that in the range from 100 GHz to 50 THz.
Such radiation is non-ionising and, as a result, it is particularly of use in medical applications. In medical imaging, the radiation is generally reflected from or transmitted through the patient.
Components of the sample being imaged will have a frequency dependent absorption coefficient and refractive index, thus each component of a sample subjected to radiation will leave its own characteristic fingerprint in the detected radiation. Thus, researchers have attempted to image samples using a plurality of frequencies to create an image from spectral information.
Measurements have been made using both frequency domain techniques, (where the amplitude of each frequency components is analysed), and time domain techniques, (where the radiation is analysed as a function of the delay time introduced by the sample into the path of the radiation). Time domain imaging is described in earlier patent GB 2 347 835.
For imaging, in the time domain, a time domain spectra is obtained for each pixel of the sample. It is then necessary to obtain a single parameter from each spectra to plot for each pixel. Previous attempts at producing images from such spectra have used the amplitude of the highest maximum or lowest minimum.